Why is he trying to touch his toes or the ground with an injured back?
I tend to think that you should be slowly trying to regain range of movement with an injured part of the body, but the danger of stretching an injury is that you don't allow it to heal properly. I've had a bullet-proof lower back my whole life and and I can't come near touching my toes. There aren't any movements in tennis that require bending over at the waist to touch the ground with your legs straight. (Bending over to pick up the ball doesn't count.)
My advice would be to back off of the tennis practice, stop the stretching (which can round the back and irritate the lower spine and muscles) and work on strengthening the core doing things like side bridges, bird-dogs, etc. But these should only be done in the absence of pain.